support connecting to unix domain sockets

[slingamn]

Including http+unix URLs --- nice potential enhancement for debugging

[slingamn]

We already support this for IRC connections, e.g. ircdog /tmp/oragono_socket works as expected.

Supporting websockets is nontrivial. It's vaguely possible to hack this into Gorilla, but as it turns out, url.Parse doesn't like http+unix URLs with percent-encoded hostnames, so we get tripped up here:

ircdog/ircdog.go

Lines 106 to 110 in 71e3129

	u, uErr := url.Parse(host)
	if uErr != nil {
	err = fmt.Errorf("Invalid host: %w", uErr)
	return
	}

Probably not worth it at this time.